This invention relates to a rheometer incorporating a blade mounted for rotation about an axis.
Rheometers are well known and used in a wide variety of chemical and material processing industries for assessing characteristics, such as flow characteristics, of materials such as powders, liquids and semi-solids such as pastes, gels, ointments and the like.
One particular rheometer is described in WO-A-9736162 in which blades extend substantially radially from a rotor shaft and are disposed at an angle relative to the axis of the shaft. The blades may be of twisted form; however the blade is at an angle over its entire length, relative to the axis of the shaft.
We have found that, although this arrangement of the prior art is generally satisfactory, some disadvantages can arise with regard to achieving repeatability when testing materials of different bulk density and rheology and particularly when comparing data derived from blades of different sizes.
It is therefore an object of the present invention to provide a rheometer incorporating a blade which eliminates or at least ameliorates the above disadvantages.
According to the present invention there is provided a rheometer incorporating a blade mounted for rotation about an axis, wherein the blade is of twisted form such that:
a first region of the blade substantially at the axis of rotation has a first angle formed by its surface with respect to a plane perpendicular to the axis of rotation such that the surface of the blade in the first region extends substantially parallel to the axis of rotation; and
a second region of the blade spaced from the axis of rotation has a second angle, different to the first angle, formed by its surface with respect to the plane perpendicular to the axis of rotation.
The blade may extend substantially at right angles to the axis of rotation, preferably substantially horizontally.
The second angle may vary with distance from the axis of rotation. For example, the second angle may decrease progressively with increasing distance from the axis of rotation.
The second angle may have a natural tangent which is inversely proportional to distance from the axis of rotation.
The rheometer may include means such that, in use, for each revolution of the blade, the blade is adapted to be displaceable by a predetermined lead distance along the axis of rotation, the second angle having a natural tangent defined as the predetermined lead distance divided by the distance of the point from the axis of rotation.
The height elevation of the twisted blade may be in mathematical proportion to blade lead distance, the blade lead distance being defined as the displacement of the blade along the axis of rotation for each revolution of the blade.
Alternatively, the height elevation of the twisted blade may be in mathematical proportion to blade diameter.
As a further alternative, the height elevation of the twisted blade may be in mathematical proportion to a ratio of blade lead distance, to blade diameter, the blade lead distance being defined as the displacement of the blade along the axis of rotation for each revolution of the blade.
The blade may be provided such that, prior to twisting, it has a progressively increasing width with increasing distance from the axis of rotation, whereby a substantially constant height elevation of the twisted blade is obtained.
The blade of the present invention demonstrates enhanced geometry compared with the prior art, enabling optimisation of performance and improved repeatability to be achieved in rheometers when used for testing materials of different bulk density and rheology. Data derived from blades of different sizes can be more reliably compared than hitherto.
For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: